1. Paffenbarger Research Center, American Dental Association Foundation, Polymers Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA; 100 Bureau Drive, Stop 8546, American Dental Association Foundation National Institute of Standards and Technology, Gaithersburg, MD 20899-8546, USA;
2. Paffenbarger Research Center, American Dental Association Foundation, Polymers Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA